There are many situations in which a person is required to show credentials to gain access to a secure or restricted area maintained by another person or entity. There are countless examples, including security doors with electronic locking mechanisms, events where admission is limited to those with tickets, special members-only lines or areas, airport security, etc. In these situations, one party generally desires access (a prospective entrant) and another party generally regulates access (a gatekeeper). The prospective entrant is typically a person or group of people. The gatekeeper may typically be a person or a business entity or the gatekeeper may be electronic (not actively stationed or monitored by a human being).
A prospective entrant generally presents a photo ID, membership card, or paper ticket. The gatekeeper verifies the credentials supplied by the prospective entrant and permits/denies entrance. There are many disadvantages to this approach, including the fact that the approach requires separate credentials supplied by the prospective entrant. These credentials can be easily lost. If the gatekeeper is a human being, access may be granted accidentally based on human error. Even where the gatekeeper is electronic (e.g., electronic card/badge reader), the access system requires specialized hardware that can be expensive, not readily available, and difficult to set up in smaller scale and/or custom, on-the-fly implementations.